﻿namespace Molten.Utility;

public static class HashHelper
{
    const int Prime = 16777619;

    // Table of prime numbers to use as hash table sizes. 
    // The entry used for capacity is the smallest prime number in this array
    // that is larger than twice the previous capacity. 

    public static readonly int[] Primes = {
        3, 7, 11, 17, 23, 29, 37, 47, 59, 71, 89, 107, 131, 163, 197, 239, 293, 353, 431, 521, 631, 761, 919,
        1103, 1327, 1597, 1931, 2333, 2801, 3371, 4049, 4861, 5839, 7013, 8419, 10103, 12143, 14591,
        17519, 21023, 25229, 30293, 36353, 43627, 52361, 62851, 75431, 90523, 108631, 130363, 156437,
        187751, 225307, 270371, 324449, 389357, 467237, 560689, 672827, 807403, 968897, 1162687, 1395263,
        1674319, 2009191, 2411033, 2893249, 3471899, 4166287, 4999559, 5999471, 7199369};

    /// <summary>Tests where or not a number is a prime. False if not.</summary>
    /// <param name="candidate"></param>
    /// <returns></returns>
    public static bool IsPrime(int candidate)
    {
        if ((candidate & 1) != 0)
        {
            int limit = (int)Math.Sqrt(candidate);
            for (int divisor = 3; divisor <= limit; divisor += 2)
            {
                if ((candidate % divisor) == 0)
                {
                    return false;
                }
            }
            return true;
        }
        return (candidate == 2);
    }

    public static int GetPrime(int min)
    {
        if (min < 0)
            throw new IndexOutOfRangeException("Min cannot be less than 0.");

        for (int i = 0; i < Primes.Length; i++)
        {
            int prime = Primes[i];
            if (prime >= min)
                return prime;
        }

        // Outside of our predefined table. Compute the hard way. 
        for (int i = (min | 1); i < int.MaxValue; i += 2)
        {
            if (IsPrime(i))
                return i;
        }
        return min;
    }

    public static int GetMinPrime()
    {
        return Primes[0];
    }

    // Modified FNV Hash in C#
    // http://stackoverflow.com/a/468084
    /// <summary>Generates a simple but generally unique hash key using FNV hashing, from a set of data, which
    /// can be used as a fast key for collections or comparisons.</summary>
    /// <param name="data"></param>
    /// <returns></returns>
    public static int ComputeFNV(params byte[] data)
    {
        unchecked
        {
            int hash = (int)2166136261;

            for (int i = 0; i < data.Length; i++)
                hash = (hash ^ data[i]) * Prime;

            hash += hash << 13;
            hash ^= hash >> 7;
            hash += hash << 3;
            hash ^= hash >> 17;
            hash += hash << 5;
            return hash;
        }
    }
}